The guideway girder is a key supporting component that ensures the safe and stable operation of high-speed maglev systems. Taking the Shanghai high-speed maglev demonstration line as the background, this paper conducts a comprehensive experimental and numerical investigation on the dynamic characteristics of the guideway girder. Dynamic response tests were carried out under an operational load corresponding to 300 km/h, and by employing the Eigensystem Realization Algorithm (ERA) together with deflection curves derived from the conjugate beam method, a complete set of dynamic properties of the guideway girder was simultaneously identified using field test data. The identified parameters were then used to calibrate a highly reliable vehicle-guideway coupled dynamic model, which showed excellent agreement with the measured data in both the time and frequency domains, thereby undergoing rigorous validation. Numerical simulations based on this model revealed a fundamental transition in the governing dynamic mechanisms of the system: the system response shifted from being primarily resonance-driven to being dominated by inertial forces and dynamic amplification due to moving loads, leading to an acceleration growth rate far exceeding that of quasi-static deflection. This study provides the maglev system with a validated model and new insights into its speed-dependent dynamics, while also pointing out directions for future research focused on mitigating high-frequency vibrations.